We bonded a wooden box to support the area around the hood blisters.We knew that we would want to support the fiberglass before we poured any concrete to prevent the fiberglass sagging and deflecting from the weight of the concrete.In the areas that we would be clamping we added some steel reinforcements before the concrete was poured
We built another wooden form for the opposite side of the fiberglass .It was thought that we could use a medium density urethane pour foam to support the weight of the concrete form while we waited for it to cure.Our first attempt didn't work because we miscalculated (guessed) the amount of material needed and tried to mold it in two pours.
After removing the failed foam with a saw we put down a layer of plastic sheet on the fiberglass to prevent the foam from sticking to the fiberglass and reset the wooden form on fiberglass.This time we had enough material mixed to do it in one pour.This time it turned out well.This part is only used to support the weight while the mold is inverted for the concrete pour.
Mark asked about using a clamping plate.This is what we built to hold the center of the panel down .